my $ip4 = "([0-9]{1,3}\.){3}[0-9]{1,3}";

my $ip6 = "([a-f0-9]{4}\:)+?";

my $prefix = "/\d\d";

##</code><code>##

my $pattern = "^($ip4|$ip6)($prefix)?";

##</code><code>##

open (FILE, "<$some_file") or die "Error: $!\n";

while (<FILE>) {

    next if !~ m[$pattern];

    print "Hey, I found \$1: $1 \$2: $2 \$3: $3\n";
}
close FILE;

##</code><code>##

^(([0-9]{1,3}\.){3}[0-9]{1,3}|([a-f0-9]{4}\:)+?)(/\d\d)?

##</code><code>##

"[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}"